There are several memory related threads on this forum. This is a nice collector thread: Minecraft Server Memory Leak - #2 by iMelsom
TLDR: Your memory usage builds up because minecraft loads world chunks into memory to make the server quicker. Free memory is unused memory, and thus also wasted memory. Trust your OS th handle memory, unless you are actually having memory issues.
Or as @hexparrot said it (in an answer to someone else) :
Question: if you’re okay allocating 7282 MB of RAM to Minecraft, why would you want Minecraft using any less than that?
Good performance comes from Minecraft having and utilizing lots of available RAM, not from having and reserving unused RAM.